STRIDERS ALMANAC – The Express Edition. Volume 1, part 2, 11 March 2013.
This is the second part of the record contained in my notebook number 200. It covers the period from 11 to 30 October 2012. This is a period of 20 days.
Part 2 IT GROWS HOT AND THE SUN PASSES OVERHEAD
11 October. There was cloudiness from Noon onwards and I heard thunder at 1340 hrs. It seemed as if there were scattered storms about. At night I saw lightning from a storm over the radio-active hills at Mount Bundey on the Arnhem Highway. There was a very little rain here. Just enough to put a trace of rain in the gauge .
12 October. The night 11-12 October was the warmest night for the season so far. There was a cooling wind at 0100 hours (both here and in Katherine it seems) but by 0651 hrs it was 24.5 degrees C here in the room at Starshine. It was overcast at dawn. By 0846 hrs it was breezy with E. Breezes. Nocturnal wind is unusual at this time of year. It seems as if the South East Trade Wind returned in the night, and as if there was an element of compression warming in the very high air temperature at dawn on the morning of the 12th.
The 13th was also a breezy day, with a S.E. wind. There was a clear return of the S.E. Trade Wind and drier conditions. It was a hot day followed by a cooler night. The night 13-14 was very cool.
15 October. The Dark Moon was at 2135 hrs on this day. There was an obvious change in the weather on the morning of this day when cloud arrived from the N.E. soon after dawn. I noticed that the Billy Goat Plum Trees (Terminalia ferdinandiana) were re-leafing in many places on this day.
16 October. At 0510 hrs a Chop Chop Nightjar called to the E.. It was a much warmer morning with the air in the room at 24.5 degrees C again. At 0540 hrs there was lightning in the E. sky and cooler air was falling down out of the E. sky. It bent the candle flame over and the air temperature fell to 23.5 degrees C.. At 0547 hrs the Kookaburras began to call in several directions. At 0554 hrs I could hear Magpie Geese talking in the far distance. I was in between being asleep and being awake and for a moment or two I thought that I was listening to a far distant frog chorus. When I woke fully I realised that I was listening to the geese muttering in the mango orchards. I woke up again at 0854 hrs. The temperature then was 28 degrees C.
At 0931 hrs there was a lovely cooling breeze as the wind got out of bed for the day. There were some small cumulus clouds at that time. At 1014 hrs there was a seabreeze (?) up the creek, from the W.. The temperature then was 30.5 degrees C.
Midday cloudiness developed and calms alternated with light E. breezes. A stronger breeze came up the creek at 1123 hrs when the air was at 31.5 degrees C. There were some brief periods of cloud shadow.
There were storms about in the afternoon and a few drops of rain fell at Solar Village. (4 mm of rain fell at Lakewood.) A very warm night followed. Very warm indeed. It was too warm for me to wear sox to bed.
According to the Television News the next day, it was a very hot night all over the Australian continent, on the night 16-17 October 2012.
17 October. Was very similar to the previous day but there was less cloud. The Terminalia ferdinandiana trees looked splendid in their new leaves, on this day.
18 October. At Lakewood there was a brief shower of rain at 0420 hrs and a more substantial fall (accompanied by thunder) at 0530 hrs. All told 4 mm of rain fell there. There was no rain at Solar Village.
On the afternoon of the 22nd there were isolated thunderstorms about and I heard thunder. There were some cooling downdrafts from a storm at Solar Village.
23 October. Thunder at dawn. There were also isolated afternoon and evening thunderstorms, and in the evening and at night lightning flickered in the sky over Mount Bundey (as it had on the previous night too). At 0013 hrs on the morning of the 24th there was a wind squall with a thunderstorm at Starshine, but very little rain fell from it. In the early afternoon of the 25th I found 2.5 mm of rain in the gauge at Solar Village.
There was a dramatic change in the weather shortly before sunset on 23 October when low and fat cumulus cloud began to blow in from the N.E.. This cloud obscured my view of the storm cloud going up over Mount Bundey. The cloud from the N.E. was still blowing over on the morning of the 24 th.
On the morning of the 25th I was in Darwin and quite big cumulus clouds were blowing over from the N.E. shortly after dawn. It remained very cloudy until about Noon when the sky cleared to reveal scattered thunderstorms. On this day the Sun was directly overhead at Noon in Darwin. It was hot.
26 October. On this day the Sub Solar Point was at the Solar Village, and I was at the Noonamah Tavern to observe the Sun at the Zenith at Noon there with some friends. The Sun did shine on us at Noon but it was a very cloudy day overall.
Rain fell from a thunderstorm at Solar Village from 1745 to 1845 hrs. There were some very welcome cooling downdrafts from that storm and 28 mm of rain fell from it. This was only the second “real rain” for the season. The previous one was 17 mm of rain at the Full Moon on 30 September. That was a Full Moon Rain, and this one was a Sub Solar Point Rain.
There was some run-off from this rain on tracks and along drains, and in the bed of the creek. There were some pools of water in the creek when it was over. In other places there was no run-off at all and the rain was absorbed by the soil.
28 October. At 0650 hrs there was thunder and rain began to fall. There was no wind with the rain. It eased to drizzle at 0730 hrs, and then resumed again. All told 44 mm of rain fell. Once again run-off was confined to tracks, drains and the creek bed. A few puddles were created in the creek with a big one under the road bridge. A cloudy day followed and there was thunder at night.
On the 29th there was a lot of re-leafing going on. White Berry Bush, Antidesma ghaesembilla,
Vitex trifolia, and Snakeweed. I also noticed a lot of freshly germinated seedlings on this day.
30 October. It was a cloudy day. I spent it getting organised and packing up to go into hospital for an operation. For the next three weeks or so I did not make any notes in my notebook.
The Trade Wind Burst
The pattern of events from the night of 12-13 October through to just after dawn on the 15th is an interesting one. There was a return to ’dry season’ (or ‘continental’) conditions with drier air and a greater daily temperature range. It often happens that the first night of such a trade wind burst is very warm or hot. The heat is due to compression warming. The air is both heated and very dry. This air will sustain very vigorous fires. Such a night is often followed by a few very hot and breezy days.
On this occasion the wind was from the E. on the morning of the 13th but it had moved around to the S.E. by the 14th. This wind shift is a common feature of these trade wind bursts.
The initial hot night that marks the start of this sequence of events should serve as a warning to the fire managers of what is about to unfold. These conditions that are so favourable to fire can be used to get a burn in some ‘difficult’ places that you want to burn such as floodplains with some green grass or areas where the soil is still moist.
It is a bit paradoxical that the start of a run of cooler nights should start with, and be signalled by, a hot night. The drop in relative humidity that is associated with the compression warming should make it possible to pick out the start of these events in the record of past meteorological observations. It should be possible to detect the signal in the 0900 hrs relative humidity records.
The Bonus Rains
It is not uncommon for there to be very little or no rain in the time-span between the Spring Equinox and the transit of the Sub Solar Point over Darwin in late October. When the Sub Solar Point passes over Darwin it also crosses the North coast of the Top End, and then the Sun is over the Australian Continent rather than over the sea to the North of it. This is a major seasonal shift in the forcing mechanisms that lie behind our day to day weather. The season of Australian Summer Cloudiness begins when the Sub Solar Point moves onto the land in the Top End. Rain is much more likely to fall after the Sun has moved inland from the coastline.
The actual day of the transit of the Sub Solar Point near Darwin tends to be cloudy. A band of cloudiness travels with the Sub Solar Point as a consequence of the massive in-pouring of energy when the Sun is directly overhead. It often rains at the time of the transit. In many years the first thunderstorm, and the first substantial rain, happens on the day of the transit, or a day or two later. In these cases the rain usually falls from a big and intense late afternoon thunderstorm.
I am inclined to think of any rain that we falls between the Spring Equinox and the day of the transit of the Sub Solar Point over Darwin as a bit of a bonus. A gift from the Gods as it were. On that basis we didn’t do too badly in 2012
Depending upon the thickness of the leaf litter layer, and the rate at which the rain falls, anything between the first 3 to 12 mm of a rainfall will be completely absorbed by the leaf litter and hardly a drop will find its way through to the soil beneath it. For this reason we can discount all of the cases or drops of rain and traces in the gauge as fairly irrelevant to what is going on in the soil.
The fall of 17 mm on 30 September would have put at least 5 mm (and possibly as much as 14 mm) of rain into the soil. The falls of 1 mm (4 October) and 4.5 mm (8 October) were probably retained in the leaf litter. The same can be said for the 2.5 mm fall on the morning of the 24th. These falls must have been important to the soil in bare places and drains, and on the edge of the bitumen along the sealed roads, but in other places their chief significance would be for the life within the leaf litter layer.
I think that we were a bit unlucky here this year. There were storms about on 7 days (in the 15 day period 11 to 25 October ) but we only got one light rainfall. Sure the amount of rainfall at any one place is a fairly random event at this time of the year, in most years, but, I think that we were bit unlucky this time. Most other places in the district probably did better than us.
The Sub Solar Point Rains
We did have a substantial fall of rain (28 mm) from a late afternoon rain with (very welcome cooling downdrafts) on the 26th of October ; the day of the transit of the Sub Solar Point over Solar Village. It was the classic archetypal event. And it was right on time. It was a bit light on in my opinion. 40 or 60 mm of rain would have been better, but the 28 mm was very welcome for all that.
The strong downdrafts that are commonly associated with the late afternoon Sub Solar Point transit thunderstorm can be used to burn some of those ‘difficult’ places like floodplains. The storm and the downdrafts may fail to materialise in any particular year but it may still be worth making a contingency plan to take advantage of the downdrafts just in case they do happen. In my opinion this is a case where informed opportunism is in order, and as the boy scouts say, it pays to be prepared. This storm can provide a valuable opportunity to put the finishing touches to firebreaks that are intended to stop the spread of fires started by lightning in November and December. It is often the case that the weak link in a firebreak is a creek crossing, a floodplain, or a swamp. The very strong downdrafts ahead of the storm can make it possible to burn these places, and with any sort of luck, the rain from the storm will put the fire out a few minutes later.
As it happened we did not have to wait very long for a bigger fall of rain. A thunderstorm on the morning terminator (the line between night and day) on the 28th of October delivered 44 mm of rain here at Solar Village. It put a big puddle, or pool of water, into Horns Creek under the road bridge due to the run-off from the roadside drains. These two Sub Solar Point rains gave us a total of 72 mm of rain on 2 rain-days over a 3 day period. This was enough rain to start the growing season in earnest, and a dramatic burst of growth and re-leafing was observed on 29 October, just one day after the 44 mm of rain fell on the morning of the 28th.
And that my friends, is how the growing season began in late 2012.
C Strider, Solar Village, 2013. (2nd draft 11 March 2013.)
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